The present invention relates to a stencil printer of the type including a plurality of ink drums.
Japanese Patent Laid-Open Publication No. 7-17121, for example, teaches a stencil printer capable of printing a multicolor image on a paper by conveying the paper only once. The stencil printer taught in this document includes a plurality of ink drums spaced from each other in a direction of paper conveyance. A particular master is wrapped around each of the ink drums. An intermediate conveyor is positioned between the ink drums for conveying a paper carrying an image printed by an upstream ink drum toward a downstream ink drum. The intermediate conveyor conveys the paper at a constant speed while each ink drum rotate at a constant peripheral speed in synchronism with a paper feed timing. In this configuration, the paper coincides with an ink image formed on each drum at a print position where the ink drum is located.
A problem arises in the stencil printer of the type described when the size (overall length) of a paper is greater than -the distance between an upstream print position where the upstream ink drum is located and a downstream print position where the downstream ink drum is located. The drums are driven by a motor or similar drive source via a drive transmission mechanism including gears and belts. The peripheral speeds of the ink drums are not always equal to each other due to the expansion or contraction of the belts or various kinds of irregularities including the dimensional errors of the gears and those of the ink drums. When the peripheral speeds of the ink drums are different from each other, it is likely that the paper is pulled or slackened in the direction of paper conveyance. For example, assume that the peripheral speed of the downstream ink drum is higher than the peripheral speed of the upstream ink drum. Then, so long as the overall length of the paper is smaller than the distance between the two print positions, the leading edge of the paper successfully reaches the downstream print position after the trailing edge of the paper has moved away from the upstream print position. The paper is therefore conveyed by the peripheral speed of the downstream ink drum. However, if the overall length of the paper is greater than the above distance, then the paper extends over both the upstream print position and downstream print position. As a result, the paper is pulled in the direction of paper conveyance due to the difference between the peripheral speeds of the two ink drums while printing is under way at the upstream print position. This is apt to cause an image to be dislocated relative to the paper at the upstream print position in the direction of paper conveyance or relative to an image to be printed at the downstream print position, resulting in a defective printing.
On the other hand, when the peripheral speed of the downstream ink drum is lower than the peripheral speed of the upstream ink drum, the paper slackens on the intermediate conveyor. This also results in a defective printing although the deviation of the image printed at the upstream print position relative to the image printed at the downstream print position will be reduced, compared to the above condition.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 64-18682, 5-229243, 6-32038 and 10-315601 (corresponding to U.S. patent application Ser. No. 09/079,287 filed May 15, 1998), and Japanese Patent Application Nos. 9-321702 and 10-167322 (corresponding to the pending U.S. patent application Ser. No. 09/164,372 filed Oct. 1, 1998).